How to rig a robotic mesh?
What I would do is:


Join all the different pieces of the robot with ctrl J, no reason for separate pieces.
shift ctrl alt C to make Origin to Geometry so that it's clean (same for armature).
In Edit mode select all and ctrl N to recalculate the Normals.
Back in Object mode, in the modifiers remove the Mirror which seems useless.
Unparent the object with alt P.
Delete all vertex groups in Data > Vertex Groups > menu available with the down arrow on the right of the Vertex Groups list > Delete All Groups
Display the layer with the armature, and even put it on the same layer than the object.
Align the bones to the mesh shape, for example drag the Spine head up to the bottom of the head.
Delete some useless bones, for example I don't think your Pelvis will be useful as I guess you don't want the hip to be flexible as it is a robot.
Perhaps create a bone for the hip that will be independent from the torso, you can use the current Spline to make the Hip, so create a child of this Hip for the torso, etc... it really depends on how you want to animate your object, which part is flexible etc...
Your armature is incomplete, for example you should have some IK controllers an pole targets for your forearms and shins, and also a root for the whole armature that would be the parent of the spine and all those additional controllers/targets
Anyway, let's make a basic parentage: As it is a robot, you won't use With Automatic Weights because it is rather made for flexible objects. Use ctrl P > With Empty Groups, it will create one Vertex Groups by bone, but it won't assign any of those Vertex Groups to the mesh yet.
Now you can manually assign each part of your mesh to the Vertex Group you want. Select your Object, go in Edit mode, select the left hand with L, go to Data > Vertex Group, with the down arrow menu Sort by Name, choose Hand.L and click Assign. Now when you move the Hand.L bone, the mesh follows.
Continue with the whole mesh.